Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the total cost to the end-user. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), direct sequence code division multiple access (DS-CDMA), orthogonal frequency division multiplexing (OFDM), orthogonal frequency division multiple access (OFDMA), and multi-carrier code division multiple access (MC-CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique modulation code, a sub-carrier constellation, or a combination thereof.
DS-CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data in assigned channels that correspond to specific unique codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, modulated by a M-ary orthogonal modulation, and spread prior to transmission by the mobile station. The data symbols following interleaving may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation of DS-CDMA is found in the TIA IS-95 CDMA standard. Another implementation is the TIA/EIA IS-2000 standard.
MC-CDMA technology is another technology used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In one example of a MC-CDMA system, convolutionally coded, formatted and interleaved user data is copied into N parallel subcarrier paths. Each of the N identical data bits is modulated by a single chip belonging to a spreading code of length N. Each data symbol simultaneously is applied to an Inverse Fast Fourier Transform (IFFT) function to modulate a different subcarrier with binary phase shift keying (BPSK). The subcarriers are separated by M/Tb Hz, where M is an integer and Tb is the modulation symbol duration. Users are identified by different modulation codes in the frequency domain. Different users transmit on the same set of subcarriers, but with different spreading codes, in the frequency domain. In some embodiments, control and pilot signals may modulate another set of subcarriers by the method described above.
In another example of a MC-CDMA system, convolutionally coded, formatted and interleaved user data is first copied into N parallel paths, where each copy of the data bit is multiplied by a chip of an N-chip spreading code assigned to the specific user. The set of N data chips is mapped by pairs into an in-phase (I) chip stream and a quadrature (Q) chip stream for QPSK modulation of each of N/2 different subcarriers. The subcarriers are combined in a combiner and then amplified and transmitted. Users are identified by different modulation codes in the frequency domain. Different users transmit on the same set of subcarriers, but with different spreading codes, in the frequency domain. Control and pilot signals also may be transmitted in this manner.
Upon reception of the MC-CDMA modulated RF signal, the received complex signal envelope is sampled and converted into a set of subcarriers using a Fast Fourier Transform (FFT). If the MC-CDMA modulation uses QPSK or QAM, the received RF signal is converted into an I-signal stream and a Q-signal stream prior to sampling and subcarrier conversion using the FFT. Each chip in the spread data stream is integrated over a symbol period to recover the symbol value. The output from each of the integrators is then converted from parallel format to serial format and the serial data stream is multiplied by the spreading code to recover the original user data stream. If the transmitter added a guard band, it is removed prior to the FFT stage.
In order to increase the reliability of CDMA receivers, base stations and wireless terminals frequently transmit M copies of the same signal, staggered in time, to the other device. The receiving device typically uses multiple receive paths, such as in a Rake receiver, to capture each of the copies. The captured copies are summed to produce a composite signal in order to improve the signal-to-noise ratio (SNR). This allows the composite signal to be more easily de-spread and recognized by a signal correlator or matched filter. However, this approach requires a large number of components and a large circuit area. Additionally, the repeated transmission of M copies of the same signal is wasteful of scarce bandwidth.
Furthermore, wireless digital communication systems increasingly are using multicarrier CDMA (MC-CDMA) and orthogonal frequency division multiplexing (OFDM) CDMA. In OFDM-CDMA, different wireless terminals (or mobile stations) are allocated different frequency spreading codes. The advantage of OFDM-CDMA is that the number of codes assigned to each wireless terminal is adjustable, leading to different data rates for different wireless terminals. However, the fact that each wireless terminal must transmit its signal over the entire spectrum leads to an averaged-down effect in the presence of deep fading and narrowband interference.
U.S. Pat. No. 6,683,908 to Cleveland disclosed an apparatus and a method that eliminate the need to transmit M copies of the same signal in order to improve signal reception. The teachings of U.S. Pat. No. 6,683,908 are hereby incorporated by reference into the present application as if fully set forth herein.
The apparatus and method of U.S. Pat. No. 6,683,908 eliminate the need to transmit M copies of the same signal by storing in memory an original copy of the received signal and generating pseudo-replicas using the stored samples of the original received signal. Each pseudo-replica is generated by randomly interchanging samples of the original received signal that occurred during time slots of the original received signal that correspond to Logic 1 and by randomly interchanging signal samples that occurred during time slots of the original received signal that correspond to Logic 0. The original signal and one or more pseudo-replicas are then combined to form a composite signal that has an improved signal-to-noise ratio (SNR).
The SNR is improved because noise in communication systems is not coherent and tends to cancel when the pseudo-replicas are repeatedly added together. But the signal is coherent and the signal components tend to add together as the pseudo-replicas are repeatedly added together. Thus, the SNR improves.
However, the apparatus and method of U.S. Pat. No. 6,683,908 operate on samples from time domain signals. This is not ideally suited for the frequency domain signals that are present in MC-CDMA and OFDM-CDMA receivers.
There is therefore a need in the art for improved multi-carrier CDMA communication systems that have an improved signal-to-noise ratio in the receiver. In particular, there is a need for multi-carrier CDMA communication systems that are not required to transmit multiple copies of a signal in order to improve SNR in the receiver.